PROJECT SUMMARY: Up to 40% of later-born siblings of children with autism spectrum disorder (ASD) are diagnosed with: a) ASD (7-19%)1,2, which is defined by social communication impairments; or b) other language and/or cognitive delays (14-20%)3 by age three. The diverse language outcomes of these high-risk (HR) toddlers and the ability to observe them prior to overt symptom emergence make them an ideal sample from which to learn about sources of variability in language development and potential risk indicators of language delays and disorders. Vocal complexity seems to be a strong predictor of later expressive language in many populations, yet this construct has been understudied.4-13 For example, aspects of vocal complexity have been found to predict later expressive language in typically developing (TD) and ASD children, but no studies have examined this relation for HR toddlers. Also, previous studies of young children's early vocalizations have not distinguished between vocal complexity occurring within preverbal (non-word) vocalizations versus that occurring within words. Doing so may result in measures of vocal complexity that are sensitive to change across the second year of life. Low vocal complexity may signal early language difficulties and need for intervention in HR toddlers.7-13 Further, vocal complexity may be malleable with intervention. Improving Parents as Communication Teachers (ImPACT) is a 12-week, naturalistic developmental behavioral intervention (NDBI) designed to improve social communication and language skills in toddlers with ASD.14-15 ImPACT teaches parents to respond contingently to toddlers' vocalizations. Because parent responsiveness to child vocalizations has been found to predict increased vocal complexity and better expressive language in TD and ASD children,16-18 it is plausible that ImPACT will also increase HR toddlers' vocal complexity. This project will examine vocal complexity within an ongoing NIDCD-funded longitudinal randomized controlled trial (RCT) of ImPACT (compared to treatment as usual) in 12-27-month-old HR toddlers. The project will add a low risk (LR) sample and measures of vocal complexity and parent contingent responsiveness. It will use longitudinal modeling and mediation analysis to examine 1) the growth of word vs. non-word vocal complexity and its relation to expressive language outcome in HR and LR toddlers; and 2) the role of the ImPACT intervention on a) parents' contingent responsiveness, b) toddlers' subsequent vocal complexity, and c) toddlers' expressive language outcome. This project will provide insight into the role of vocal complexity as a risk indicator of ASD and language delay as well as its malleability in response to a parent-implemented social communication intervention. The proposed research and training plan will provide experience and mentorship to a talented and motivated young researcher, and it will be crucial for the applicant's development as an independent and productive clinical scientist.